Patent application title: System and method for reducing critical current or magnetic random access memory

Abstract:

A system and a method for reducing critical current of magnetic random
access memory (MRAM) are disclosed. The magnetic device includes at least
a pinned layer, a spacer layer and a free layer, and the material of the
pinned layer and the free layer is perpendicularly anisotropic
ferrimagnetic. The spacer layer is an insulator. By the modified
Landau-Lifshitz-Gilbert equations, the varying trend of the critical
current can be estimated.

Claims:

1. A method for reducing critical current of a magnetic random access
memory, comprising:using modified Landau-Lifshitz-Gilbert equations to
derive an intermediate formula describes the dynamics of net
magnetization;calculating the dynamics of net magnetization by the
intermediate formula under the influence of a spin-polarized current to
derive a resultant formula, wherein the spin-polarized current is
arranged to apply to the magnetic random access memory; andinputting the
boundary conditions of the magnetic random access memory into the
resultant formula to obtain a value of the critical current.

2. The method of claim 1, wherein the modification of the modified
Landau-Lifshitz-Gilbert equations is provided by involving effective
parameters.

3. The method of claim 1, wherein a value of the critical current is
decreased by changing a spin orientation of the spin-polarized current.

Description:

RELATED APPLICATIONS

[0001]This application is a Divisional/Continuation patent application of
co-pending application Ser. No. 11/645,550, filed on 27 Dec. 2006. The
entire disclosure of the prior application Ser. No. 11/645,550, from
which an oath or declaration is supplied, is considered a part of the
disclosure of the accompanying Divisional/Continuation application and is
hereby incorporated by reference.

BACKGROUND

[0002]1. Field of Invention

[0003]The present invention relates to a system and a method for reducing
critical current of magnetic random access memory, and more particularly
to a system and a method for reducing critical current of a magnetic
device with perpendicularly anisotropic ferrimagnetic structure.

[0006]Reference is made to FIG. 1a and FIG. 1b, which show a conventional
magnetic memory device 100. The magnetic memory device 100 includes an
antiferromagnetic layer 110, a pinned layer 120, a spacer layer 130 and a
free layer 140.

[0007]The antiferromagnetic layer 110 is used to fix, or pin, the
magnetization of the pinned layer 120 in a particular direction. The
pinned layer 120 and the free layer 140 are ferromagnetic with a
magnetization 121 and 141 in the plane, respectively. The spacer layer
130 is a nonmagnetic insulator. The magnetization 141 of the free layer
140 is free to rotate, typically in response to an external field.

[0008]FIG. 1a shows the magnetization 121 and 141 as parallel in the same
direction. In this configuration, the magnetic resistance of the magnetic
random access memory 100 is in a lower state. FIG. 1b shows the
magnetization 121 and 141 as parallel in opposite directions, and the
magnetic resistance of the magnetic random access memory 100 is in a
higher state.

[0009]A conventional method for changing the direction of the
magnetization of the free layer is to apply two orthogonal currents to
the magnetic device, for example, the X-Y selection mechanism. The method
applies two orthogonal currents as read and write currents of each
magnetic device. Thus, either a definite volume of each magnetic device
is required, or the adjacent magnetic device in the memory device array
is affected by the read or write current.

[0010]However, there are some disadvantages in the conventional magnetic
device. For example,

[0011]1. The conventional magnetic device needs an antiferromagnetic layer
to fix the pinned layer's magnetization; the manufacturing process is
more complicated.

[0012]2. The known method of changing the magnetization direction limits
the density of the magnetic device array, thus raising power consumption.

SUMMARY

[0013]It is therefore an objective of the present invention to provide a
system that can be a magnetic random access memory, which applies
perpendicularly anisotropic ferrimagnetic material to form the pinned
layer and the free layer. There is no need for the additional
antiferromagnetic layer of the prior art to fix the pinned layer. Unlike
the prior art, the magnetization of the pinned layer and the free layer
are perpendicularly anisotropic, so the volume of the magnetic device of
the present invention can be smaller than the known one.

[0014]It is another objective of the present invention to provide a method
for reducing critical current of the magnetic random access memory. The
method employs a modified Landau-Lifshitz-Gilbert (LLG) equation that
includes spin transfer effect to simulate the variation of critical
current value.

[0015]According to the aforementioned objectives of the present invention,
a magnetic system is provided. In one embodiment of the present
invention, the magnetic system includes a pinned layer, a spacer layer
and a free layer. The pinned layer is the base layer of the magnetic
system, and the free layer is the top layer. The material of the pinned
layer and the free layer are ferrimagnetic, and both of the
magnetizations are perpendicularly anisotropic, wherein the magnetization
of the free layer is free to rotate. The spacer layer is between the
pinned layer and the free layer, and the material of the spacer layer is
insulating material.

[0016]The magnetization precession and switching (i.e. rotation) of the
free layer is induced by the spin transfer torque of spin-polarized
current, and the positive/negative spin-polarized current passes through
the magnetic system's sandwich structure, which means the electrons flow
up or down.

[0017]In accordance with the foregoing and other objectives of the present
invention, a method for reducing critical current is provided. A final
equation via the modified LLG equation is obtained to describe the
dynamics of net magnetization. The final equation shows the time
evolution of net magnetization under the influence of a spin-polarized
current, as well as the estimation of the critical current for the
practical application in MRAM writing.

[0018]Because the different spin-polarized currents have distinct spin
orientations, individual critical current and current density values are
obtained. Finally, the varying trend of the critical current is given.

[0019]It is to be understood that both the foregoing general description
and the following detailed description are by examples and are intended
to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]The accompanying drawings are included to provide a further
understanding of the invention and are incorporated in and constitute a
part of this specification. The drawings illustrate embodiments of the
invention and, together with the description, serve to explain the
principles of the invention. In the drawings,

[0023]FIG. 2 illustrates a magnetic random access memory of the preferred
embodiment of the present invention;

[0024]FIG. 3 illustrates a spin-polarized current applied to a magnetic
system of the preferred embodiment of the present invention;

[0025]FIG. 4a illustrates the spin orientation of the spin-polarized
current applied to the magnetic system (θ=0);

[0026]FIG. 4b illustrates the spin orientation of the spin-polarized
current applied to the magnetic system (θ=π/2);

[0027]FIG. 4c illustrates the spin orientation of the spin-polarized
current applied to the magnetic system (θ=π); and

[0028]FIG. 4d illustrates the spin orientation of the spin-polarized
current applied to the magnetic system (θ=3π/2).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029]Reference is now made in detail to the present preferred embodiments
of the invention, examples of which are illustrated in the accompanying
drawings. Wherever possible, the same reference numbers are used in the
drawings and the description to refer to the same or like parts.

[0030]While the specification concludes with claims defining the features
of the invention that are regarded as novel, it is believed that the
invention is better understood from a consideration of the following
description in conjunction with the figures, in which like reference
numerals are carried forward.

First Embodiment

[0031]Reference is made to FIG. 2, which illustrates a magnetic memory
random access memory of the preferred embodiment of the present
invention. A magnetic random access memory 200 includes a pinned layer
210, a spacer layer 220 and a free layer 230.

[0032]The pinned layer 210 is a base layer of the magnetic random access
memory 200. The material of the pinned layer 210 may be a ferrimagnetic
thin film, such as TbFeCo alloy, DyFeCo alloy, Co/Pt multilayer thin
film, Co/Pd multilayer thin film, or other ferrimagnetic multilayer thin
film. A dipole moment 211 and a dipole moment 212 are perpendicularly
anisotropic and represent a definite strength, form a net magnetization
of first layer 213.

[0033]The spacer layer 220 is a nonmagnetic layer, which is an insulator.
The free layer 230 is a top layer of the magnetic random access memory
200. The material of the free layer 230 could be a ferrimagnetic thin
film, such as TbFeCo alloy, DyFeCo alloy, Co/Pt multilayer thin film,
Co/Pd multilayer thin film, or other ferrimagnetic multilayer thin film.
If the free layer 230 is a TM-rich (Transition Metal; TM) material,
wherein a component of a magnetization 231 and a component of a
magnetization 232 form a net magnetization of second layer 233; if the
free layer 230 is a RE-rich (Rare Earth; RE) material, wherein a
component of a magnetization 234 and a component of a magnetization 235
form a net magnetization of second layer 236, which are perpendicularly
anisotropic and free to rotate; namely, the net magnetization of second
layer 233 and the net magnetization of second layer 236 may form an
included angle with the direction normal to the layers.

[0034]The thickness of the pinned layer 210 is 0.5 to 100 nm. The
thickness of the spacer layer 220 is 0.5 to 10 nm. The thickness of the
free layer 230 is 0.5 to 100 nm. The thickness and the composition of
every layer can be modulated to change their magnetic and electric
properties.

Second Embodiment

[0035]Reference is made to FIG. 3, which illustrates a spin-polarized
current applied to the magnetic memory device of the preferred embodiment
of the present invention.

[0036]A component of a magnetization 237 and a component of a
magnetization 238 of the free layer 230 form a net magnetization of
second layer 239, and the net magnetization of second layer 239 may form
an included angle θa with the direction normal to the layers,
namely, the net magnetization of second layer 239 substantially
perpendicular to the free layer 230.

[0037]A spin-polarized current 240 drives through the magnetic random
access memory 200 upward or downward as a read current or a write
current, which makes the net magnetization of second layer 239 turn
upward or downward (i.e. the spin transfer effect). The orientation of
spin 241 has an included angle θb with the spin-polarized
current 240, which determines the critical current value.

Third Embodiment

[0038]Referring to FIG. 3 again, modified LLG equations (1) and (2) for
the net magnetization of second layer 239 formed by the component of a
magnetization 237 and the component of a magnetization 238 are given
below, by taking the parameters into account in Table 1:

TABLE-US-00001
TABLE 1
(1)
γ ×α ×μ±γ ±
×μ×μ ##EQU00001##
(2)
γ ×α ×μ±γ ±
×μ×μ ##EQU00002##
Parameters Definitions of the parameters
M1 component of a magnetization 237
M2 component of a magnetization 238
M1 magnetization magnitude of M1
M2 magnetization magnitude of M2
γ1 gyromagnetic ratio of the component of a magnetization
237
γ2 gyromagnetic ratio of the component of a magnetization
238
H1 net effective field of the component of a magnetization 237
H2 net effective field of the component of a magnetization 238
hM1 effective local exchange field of the component of a
magnetization 237 on the component of a magnetization
238 (where h ≦ 0)
hM2 effective local exchange field of the component of a
magnetization 238 on the component of a magnetization
237 (where h ≦ 0)
α1 corresponding damping coefficient of γ1
α2 corresponding damping coefficient of γ2
μ1 unit vector of M1
μ2 unit vector of M2
μ3 unit vector of the net magnetization of first layer 213
reduced Planck's constant = h/2π
e electron charge = 1.602 × 10-19 Coulomb
V volume of the free layer 230
Ie1 spin-polarized current of electron 1 (e1)
Ie2 spin-polarized current of electron 2 (e2)
g1 coefficient for the component of a magnetization 237 which
depends on polarization of the electron 1 (e1)
g2 coefficient for the component of a magnetization 238 which
depends on polarization of the electron 2 (e2)
± positive or negative, depending on the direction of the
spin-polarized current

[0040]The θ1,2 of the formula (8) depends on the orientation of
the spin 241 with regard to orientation of the net magnetization of
second layer 239 formed by the component of a magnetization 237 and the
component of a magnetization 238.

[0041]Assuming μ3=c, Heff=Heff c (C is a constant), and
considering an antiparallel coupling effect between magnetic rare-earth
(RE) and transition-metal (TM) samples, the aforementioned intermediate
formula (3) can be solved as follows:

{dot over (θ)}=±(aIeff.sup.±-ωαeff)-
sin θ (9)

[0042]A resultant formula (9) allows obtaining the eight critical current
values of the spin-polarized current for different spin orientations,
which present in the form of the formulas (10), (11) and (12) below:

±α ω γγ ±± ± α ω
γγ±± ±α ω γγ± ±
##EQU00004##

Fourth Embodiment

[0043]Reference is made to FIGS. 4a, 4b, 4c and 4d, wherein there are
eight spin orientation configurations of the spin-polarized current
applied to the same magnetic memory device. The component of a
magnetization and the net magnetization of the free layer may have a
included angle θ with the perpendicular line and free to rotate.

[0044]For example, a Tbx(FeCo)1-x sample using M1=2644
XR emu/cm3 and M2=799(1-XR) emu/cm3, where
XR is atomic percentage of the RE element, a minimum value for both
Ic+ and Ic- when XR=24% can be found.

[0045]The Ic+,i and Ic-,i values are obtained (the
result listed in Table 2 below) by using formulas (10), (11) and (12),
which assume a 60×130 nm2 elliptical sample for a
Tbx(FeCo)1-x ferrimagnetic structure. The parameters used in
all the results mentioned are in Table 3 below.

[0046]As the value of the spin orientation θc changes from 0 to
π, the value of critical current Ic- decreases; and the current
density Jc+ also decreases. Furthermore, when the value of the spin
orientation θc changes from π to 0, the value of critical
current Ic- decreases; and the current density Jc+ also
decreases continuously.

[0047]By the manner of deriving the modified LLG equations, the variation
tendency of the critical current value can be confirmed by changing the
spin orientation. After setting some boundary conditions, the estimation
of the critical current is obtained.

[0048]According to the composition and the embodiments above, there are
many advantages of the present invention over the prior art, such as:

[0049]1. The manufacturing processes and the structural layers of the
magnetic system of the present invention are fewer than those of the
prior art, so the cost and yield of production are improved.

[0050]2. The material of the pinned layer and the free layer is
perpendicularly anisotropic ferrimagnetic, which allows the volume of a
single magnetic system to be smaller than that of the prior art.

[0051]3. By the method of controlling the spin orientation of the
spin-polarized current, the power consumption of the magnetic system can
be reduced via reducing critical current.

[0052]It will be apparent to those skilled in the art that various
modifications and variations can be made to the structure of the present
invention without departing from the scope or spirit of the invention. In
view of the foregoing, it is intended that the present invention cover
modifications and variations of this invention provided they fall within
the scope of the following claims and their equivalents.